Basically? Cannibalizing majority-minority districts for their voters. Expect to see more casual attacks on the process like this from Greg Sargent:
Today I chatted with David Wasserman, who closely tracks House districts for the Cook Political Report. Wasserman recently wrote that due to population shifts and redistricting that have resulted in huge concentrations of Democratic votes in Dem districts — wasting a lot of those votes — Democrats can now expect that the percentage of seats they win will consistently trail their victory in the overall popular vote by about four percentage points.
Bolding mine. From the point of view of the Congressmen who hold those seats, those votes aren’t wasted at all – which is why the CBC and the GOP so often find themselves on the same side of redistricting court battles. Basically, those ‘huge concentrations’ are minority voters, which is to say, black ones; and when 90% of a particular racial group consistently vote for one political party in a two-party system you tend to see operatives from both political parties take advantage of that. So we should be seeing an amusing internal struggle going on inside the Democratic party, as white Democrats attempt to bust up existing majority-minority districts and fight to stop new ones from coming into existence…
PS: Oh, one other wrinkle: black Democrats will vote for Democrats. White Democrats tend not to vote for black politicians whose name isn’t Barack Obama. So all those Congressmen in the CBC will promptly lose their jobs if their districts are redesigned, and they all know it.